1. Field of the Invention
The present invention relates to an exposure apparatus for making semiconductor circuits or the like by photo-lithography, and in particular to an exposure apparatus suitable for exposure of photosensitive substrates coated with chemically amplified resist.
2. Description of the Related Art
A projection exposure apparatus, which is called a stepper, is used in a photo-lithographic process for making semiconductor circuits, liquid crystal substrates, or the like. Because the apparatus needs to control its temperature with a high precision, it has an air-conditioner for this purpose. Because the air-conditioner needs to control the temperature within a very strict range, which may be xc2x10.1 centigrade, with respect to a set temperature, the air-conditioning system is constructed as a circulatory system. The air-conditioner needs a fan for air circulation. To avoid vibration and other problems, the air-conditioner including a fan needs to be independent of the main body including a chamber, which houses an exposure unit. FIG. 12 is a schematic top plan of a conventional exposure apparatus with an air-conditioner 1 independent of a main body 2. The air-conditioning system in this type of apparatus is described below with reference to FIG. 12.
The main body 2 has a chamber 3 housing an exposure unit, which includes a reticle R and a projection lens PL. Because an apparatus for making semiconductors needs to keep clean, there is a need for the pressure in the chamber 3 to be always positive relatively to that outside the chamber. The pressure difference between the inside and outside of the chamber 3 makes air in the chamber leak out. The leakage needs compensating by supplying air from the outside. Formed for this purpose is an outside air (OA) inlet 6 for natural intake through it to normally take in outside air.
The air-conditioner 1 includes a cooler 7 having radiation fins (not shown), a heater 8 and a fan 9. The air returning from the chamber 3 enters the air-conditioner 1 together with the air supplied from the outside. The air having entered the air-conditioner 1 is first cooled by the cooler 7. On the radiation fins condenses the superfluous moisture brought from the outside by taking in outside air. The condensate is removed as stated later. The cooled air is heated to a desired temperature by the heater 8. The heated air is sent to the main body 2 by the fan 9. A temperature sensor 11 is fitted downstream from the fan 9. The values detected by the sensor 11 are sent to a controller (not shown). Monitoring the output from the sensor 11, the controller controls the cooler 7 and heater 8 so that the difference between the output value and a target temperature is zero. Thus, the temperature is adjusted by feedback control.
The main-body 2 also includes a HEPA (high efficiency particle air) box 13 having a HEPA filter 10. To maintain the cleanliness of the air in the chamber 3, the air having passed through the air-conditioner 1 is cleaned by the filter 10 before supplied to the chamber 3.
Positioned below the cooler 7 is a drain pan 4, to which a drain pipe 5 is connected. The moisture condensed by the cooler 7 is collected by the pan 4. The collected water is drained through the pipe 5 from the apparatus.
Recently, it has been found that a very small amount of gas in a clean room atmosphere affects the reduction projection exposure apparatus or other apparatus for making semiconductors in the atmosphere. This is explained below more specifically.
Sometimes insufficient is the luminance of the light sources of excimer laser exposure apparatus with an excimer laser used as the light source, X-ray exposure apparatus, electron beam exposure apparatus, etc. To cope with the insufficiency by virtue of the high sensitivity of resist, chemically amplified sensitive resist is used. This resist contains an acid-generating agent as the photosensitive agent in it. The acid generated by exposing the resist induces catalytic reaction in the succeeding heat treatment (PEB). This accelerates the insolubilization (negative type) or the solubilization (positive type) in developer of the resist. For example, if a chemically amplified resist is positive, a very small amount of basic gas of a ppb level in the atmosphere neutralizes the acid catalyst generated on the resist surface, and forms a layer which makes the surface slightly soluble. After development by exposure, the resist cross section, which should otherwise be rectangular, has T-shaped eaves. Because a chemically amplified resist, which is a sensitive resist, cannot be used as it is, it may need overcoating, so that the throughput lowers.
As the wavelength of exposure light has been shorter and the illuminance has been higher, such a problem has been occurring that a very small amount of gas in the atmosphere deposits or precipitates as a clouding substance on the surfaces of illumination system members. This is due to the photochemical interaction between the exposure light and the small amount of gas in the atmosphere. The reactant may be ammonia gas or sulfur oxide in the air, an organosilicic compound, or the like. The clouding of illumination system members results in remarkably low illuminance, so that the throughput is reduced.
U.S. Pat. No. 5,430,303 discloses a chemical filter for removing chemical substances, as means for removing a very small amount of gaseous impurities in a clean room atmosphere.
Because a very small amount of such gas exists originally in a clean room, it is desired to remove the gas by some means before the gas enters an exposure apparatus. The removal is difficult, however, due to the structure of the air-conditioner of the apparatus. Specifically, the air-conditioner is open to the outside through the drain piping, which is essential for temperature control. In addition, the heater and cooler of a typical exposure apparatus are positioned immediately upstream from the fan because of the apparatus structure. Because of the fan characteristics, the pressure in the drain pan of the apparatus is negative (about xe2x88x9230 mmAq.) relatively to the atmospheric pressure. As a result, the following disadvantages arise.
1. The drain piping is connected finally to other devices in the semiconductor manufacturing factory where the apparatus is installed. The pressure in the drain pan is negative relatively to the atmospheric pressure. When no water stays in the pan, gaseous impurities from a manufacturing device in the factory may flow through the piping into the apparatus.
2. Chemical substances are produced in the atmospheres of other manufacturing devices by chemical processes of alkaline, acid or other treatment in the devices. Very small amounts of the substances may dissolve in the air-conditioner drainage of the devices. Such substances of a ppb level may probably flow through the drain piping into the exposure apparatus. As far as piping systems are not drains for drainage containing highly concentrated chemical substances, chemical contamination of apparatus has been given no particular consideration in many of them. The air-conditioner drain piping system of an exposure apparatus may be connected to a clean room atmosphere outside the apparatus.
3. Depending on apparatus installation conditions, condensate from the cooler of an exposure apparatus may always stay in the drain pan. In such a case, although no outside air directly enters the apparatus through factory piping, impurities in the outside air may indirectly enter through the staying condensate. A problem arises any way.
4. The condensate staying in the drain pan does not flow out for a long time due to the negative pressure. Therefore, a very small amount of gas in the circulating air dissolves in the staying condensate. After the solution is concentrated, microorganisms etc. may propagate in it. Thus, the condensate may be a secondary source of gas affecting the exposure apparatus, even if the influence of factory piping is small.
It is the object of the present invention to provide an exposure apparatus which can solve the problems in the conventional art, prevent outside impurities from flowing reversely through the drain and air-conditioner of the apparatus into the apparatus and therefore effectively restrain the production of a layer making the surface of a chemically amplified resist slightly soluble, the decrease in illuminance due to a clouded optical material, and so on.
According to the first aspect of the invention, there is provided an exposure apparatus comprising:
an exposure unit for illuminating a mask pattern to transfer an image of the mask pattern onto a photosensitive substrate;
a chamber which houses the exposure unit, and in which an atmosphere is kept in a constant condition;
an air-conditioner for adjusting the atmosphere in the chamber to the constant condition;
a drain for draining condensate produced in the air-conditioner;
trapping system for temporarily trapping the condensate before draining it; and
a liquid supply system for supplying clean liquid continuously or intermittently to the trapping system.
In the exposure apparatus of the invention, during air-conditioning in the air-conditioner, the condensate from a cooler of the air-conditioner is collected temporarily in the trapping system. At the same time, the trapping system is supplied with clean liquid such as clean water continuously or intermittently by the supply system. Therefore, a sufficient amount of water stays always in the trap, and the water in the trap keeps flowing out. consequently, the impurities dissolved in the drainage including the condensate will be diluted, so that it is possible to prevent them from being concentrated. It is also possible to prevent the impurities from being concentrated in the trap. In addition, because clean water flows always into the trap from upstream, substances contained in the water in the trap keep discharged. Therefore, it is possible to prevent impurities from entering indirectly through the piping system downstream from the trap. In other words, it is possible to prevent these impurities from being released again. In this specification, the terms xe2x80x9cclean liquidxe2x80x9d and xe2x80x9cclean waterxe2x80x9d respectively mean the liquid and water in which the impurities such as ammonia and/or microorganisms have been removed to at least a ppb level.
In the exposure apparatus of the invention, it is preferable that the drain include a drain pipe, and that the trapping system be a U-tube which is a midway portion of the pipe. The U-tube always holds the amount of liquid which is based on the pressure difference between the inside of the air-conditioner and the outside of the apparatus. The trapping system being the U-tube, it is possible to securely collect in the trapping system a constant amount of water, which is based on the difference between the inside and outside pressures, without using a special controller or the like, if clean water flows together with the condensate into the trapping system from upstream. The portion of the water in excess of the constant amount keeps flowing out.
The trapping system may be a drain pan interposed between the air-conditioner and drain. The drain pan may be structured to receive the condensate from the air-conditioner. The drain pan may be connected to the inlet end of the drain pipe. The drain pan is advantageous if the exposure apparatus is installed on a solid floor, where there is no sufficient difference of elevation for forming a U-shaped piping portion as stated above. In such a case, it is preferable that there be such an amount of water in the pan that the water surface in the drain pan is always higher than the top of the inlet end of the drain piping. This can prevent air which contains impurities from reversely flowing directly into the apparatus from the piping downstream from the drain pan. In such a case, by supplying the drain pan with clean water continuously or intermittently, polar molecules such as ammonia which are highly soluble in water dissolve in the water in the drainpan, and are discharged from the exposure apparatus. Therefore, the drain pan functions as a quasi-chemical filter.
It is preferable that the exposure apparatus comprise a detector for directly or indirectly detecting the amount of the liquid in the trapping system, and a flow controller for controlling on the basis of the results of the detection by the detector the quantity of the liquid supplied by the liquid supply system so that a substantially constant amount of liquid always stays in the trapping system. The detector for directly detecting the amount of liquid may be a level sensor for detecting the height of the water surface in the trapping system. The detector for indirect detection may be a flow sensor for detecting the flow rate of the water flowing out of the trapping system, or a flow sensor for detecting the total flow of the water, which includes the condensate, flowing into the trapping system. By thus providing the detector and the flow controller, the quantity of water supplied from the supply system is controlled so that there is a nearly constant amount of water always in the trapping system. Therefore, if the trapping system is a drain pan, it is possible to set the amount of water so that the water surface in the pan is always higher than the top of the inlet end of the drain piping. It is also possible to keep the flow rate at a certain value or higher so that no water in the drain pan stagnates.
According to the second aspect of the invention, an exposure apparatus comprising:
an exposure unit in which a mask pattern is illuminated and its image is transferred onto a photosensitive substrate;
a chamber which houses the exposure unit, and in which an atmosphere is kept in a constant condition;
an air-conditioner for adjusting the atmosphere in the chamber unit to the constant condition;
a drain for draining condensate produced in the air-conditioner; and
a filter interposed between the air-conditioner and the drain for removing impurities in the condensate produced in the air-conditioner.
In the exposure apparatus of the invention, the filter can remove impurities in the condensate, and adsorb gaseous impurities which have entered from outside through the drain channel of the drain, and therefore prevents them from entering the exposure unit. Consequently, it is possible to effectively prevent impurities from being accumulated at the drain channel inlet, where a drain pan; is fitted in general.
The filter may be a waterproof chemical filter, which may be fitted at the end of the drain channel, which is adjacent to the air-conditioner, in such a manner that the filter constitutes a partition between the inside of the air-conditioner and the inside of the drain channel. The drain channel interconnects the air-conditioner and the outside of the apparatus. It is preferable that the structure of the chemical filter be so devised or adapted that the pressure loss of the filter is set at a large value to some extent. In such a case, the filter itself is a trap, which prevents polluted air from flowing reversely from the drain pipe.
An ion sensor may be fitted to the filter. The sensor can detect the concentration of the impurities adsorbed by the filter. Consequently, it is possible to judge the filter life, and therefore judge exactly when to replace the filter.
According to the third aspect of the invention, there is provided an exposure apparatus comprising;
an exposure unit for illuminating a mask pattern to transfer an image of the mask pattern onto a photosensitive substrate;
a chamber which houses the exposure unit, and in which an atmosphere is kept in a constant condition;
an air-conditioner for adjusting the atmosphere in the chamber to the constant condition;
a trapping system for temporarily trapping condensate produced in the air-conditioner;
a detector for detecting the amount of the liquid in the trapping system;
a drain path for draining the condensate in the trapping system to the outside of the exposure apparatus;
an opening/closing mechanism provided in the drain path for opening and closing the drain path; and
a controller for controlling the opening/closing mechanism according to the amount of the liquid in the trapping system detected by the detector.
When the air-conditioner performs an air-conditioning operation while the opening/closing mechanism is closed, the condensate from the air-conditioner, especially, a cooler of the air-conditioner, is collected temporarily in the trapping system. The amount of the liquid collected in the trapping system is monitored by the detector. The controller controls the opening/closing mechanism according to the amount of the liquid in the trapping system that was detected by the detector. Normally, therefore, a reverse flow of dirty air containing pollutants from the downstream side of the drain path can be blocked by closing the opening/closing mechanism. When a certain amount of the liquid (condensate) accumulates in the trapping system, on the other hand, the opening/closing mechanism is opened to drain the liquid collected in the trapping system to the outside through the drain path. By so doing, it becomes possible to prevent an undesired degree of buildup of the pollutants in the liquid collected in the trapping system.
Depending on the environment where the exposure apparatus is installed, the humidity of the atmosphere makes a difference. This produces a difference in the amount of the liquid collected in the trapping system. According to the present invention, by contrast, the amount of the liquid in the trapping system is monitored by the detector. The controller can control the opening/closing mechanism according to this amount. Therefore, the amount of the liquid in the trapping system is not affected by the environment of installation.
The exposure apparatus,.the third aspect of the invention, further includes a cleaning device having a liquid supply system for supplying a clean liquid continuously or intermittently to the trapping system. The controller can control the opening/closing mechanism and the cleaning device, for example, in the following manner: As mentioned above, the opening/closing mechanism is opened to drain the liquid in the trapping system through the drain path. When this liquid has been drained off (or the liquid remaining in the trapping system reaches a certain amount), the controller controls the cleaning device so that a clean liquid is supplied by the liquid supply system continuously (or intermittently) to the trapping system for cleaning purposes. The cleaning liquid washes the trapping system. Even if the concentration of the impurities in the trapping system occurs somewhat, the concentrated impurities will be washed away, and can thus be prevented from being released again from there.
In the exposure apparatus as the third aspect, when a predetermined upper-limit amount of the liquid is trapped in the trapping system, the controller opens the opening/closing mechanism. When the liquid in the trapping system has been drained to reach less than a predetermined amount, the controller starts liquid supply from the liquid supply system to the trapping system. After cleaning by the cleaning device is completed, the liquid used to clean the trapping system is drained. When this liquid has been drained to reach below a predetermined lower-limit amount, the controller can close the opening/closing mechanism.
In other words, the controller opens the opening/closing mechanism when the predetermined upper-level amount of the liquid has been accumulated in the trapping system. As a result, the liquid accumulated in the trapping system is drained to the outside through the drain path. When the liquid in the trapping system has been drained to less than a predetermined amount, liquid supply by the liquid supply system to the trapping system started by the controller. As a result, the trapping system is washed with a cleaning liquid. This washing is carried out using, say, a certain amount of clean water. During this procedure, the opening/closing mechanism remains open. After cleaning (washing) by the cleaning device is completed, the liquid used for the cleaning of the trapping system is drained. At a time when this liquid has been drained to less than the predetermined lower-level amount, the controller closes the opening/closing mechanism. After that, the opening/closing mechanism is never opened until a certain amount of the liquid is accumulated in the trapping system.
According to the above-described constitution, the cleaning liquid flows through the drain path during cleaning (washing). A certain time after completion of cleaning, a flow of this cleaning liquid nearly vanishes. At a time when this has occurred, and after then, the opening/closing mechanism keeps closed. Thus, reverse flow of the impurities from the piping system downstream from the trapping system can be prevented nearly without fail.
The exposure apparatus as the third aspect may further include a pump for forcing the liquid in the trapping system into the drain path. The air-conditioner is in a vacuum in order to circulate clean air into the exposure apparatus. Thus, simply by opening the opening/closing mechanism to leave the drain path open, not all of the liquid accumulated in the trapping system may be drained promptly. Even under these circumstances, the liquid in the trapping system can be forced by the drain pump into the drain path. Because of this constitution, the dwell time of the liquid in the trapping system can be shortened, even when the vacuum of the air-conditioner is high. Consequently, the dissolution and concentration of the impurities can be minimized.
In the exposure apparatus as the third aspect, the clean liquid may be water containing a trace amount of a germicidal ingredient and substantially free from chemical impurities or microorganisms. The chemical impurities refer, for example, to nitrogen compounds which chemically change into substances, such as ammonia gas, that will cause T-shaped deformation of a chemically amplified resist upon photochemical interaction. The microorganisms include, for example, bacteria. The freedom from the chemical impurities or microorganisms refers, for instance, to the removal of them to a ppb level or less. A typical example of the germicidal ingredient is hydrogen peroxide. The use of such water can restrain the multiplication of microorganisms in the trapping system, and also prevent the chemical change of chemical impurities, such as the conversion of nitrogen compounds into ammonia, etc.
According to the fourth aspect of the invention, there is provided an exposure apparatus comprising:
an exposure unit for illuminating a mask pattern to transfer an image of the mask pattern onto a photosensitive substrate;
a chamber which houses the exposure unit, and in which an atmosphere is kept in a constant condition;
an air-conditioner for adjusting the atmosphere in the chamber to the constant condition;
a trapping system for temporarily trapping condensate produced in the air-conditioner;
a drain path for draining the condensate in the trapping system to the outside of the exposure apparatus; and
a mechanism provided in the drain path for preventing a pollutant from entering the air-conditioner from the outside of the exposure apparatus through the drain path.
In the exposure apparatus as the fourth aspect, the mechanism for preventing a pollutant from entering the air-conditioner from the outside of the exposure apparatus through the drain path may be a U-tube, and a liquid supply system for supplying a clean liquid to the trapping system; or may be a solenoid valve for opening and closing the drain path.